Flexography, which is one example of relief printing, produces an image on a substrate by transferring ink from the surface of a relief plate, representing the image, directly to a substrate. Relief features in a flexographic plate are typically formed by subjecting a plate precursor to a curing radiation (e.g. ultraviolet light) through an image-wise mask and then developing the precursor to wash away parts of the plate that have not received sufficient curing radiation. The resulting relief features typically comprise solid areas and halftone dots of varying sizes and/or quantities per area to represent a range of tones specified by the image data. For example, a highlight tone can be represented by an array of very small relief dots in an area, a shadow tone can be represented by an array of large dots in an area, and a full tone can be represented by a solid relief area.
A number of challenges exist in preparing and printing with relief plates. One challenge is to produce relief features that accurately represent image features. Another challenge is to transfer an optimal quantity of ink from relief features so that printed ink densities on the substrate have a wide range and relatively linear correlation with image tonality. Another challenge is to transfer ink with a uniform density to the substrate so that areas representing a specific image tonality have a consistent appearance. The prior art teaches a number of techniques to address individual challenges, as described below. However, similar techniques appear to produce a variety of results.
U.S. Pat. No. 6,063,546 (Gelbart) teaches the use of a mask with varying optical density to control the amount of curing radiation delivered to individual plate precursor features. In particular, Gelbart teaches that relief feature accuracy can be improved by allowing a full exposure for highlight features, and gradually reducing exposure as tonality increases to some optimal level for full tone features. Gelbart teaches an analog method for varying optical density. For example, one or more layers of UV light-absorbing mask material can be removed to provide partial transparency for an image feature in a mask. Gelbart also teaches a digital method for varying optical density. For example, Gelbart teaches an area modulation technique involving a pseudo-random distribution of opaque features in an image area of a mask to effect an average reduction in exposure for the corresponding relief feature. Gelbart teaches that these opaque features should be small enough that upon exposure and developing they are not resolved in the relief plate (e.g. as relief holes).
U.S. Pat. No. 7,279,254 (Zwadlo) teaches laminating a mask to a plate precursor prior to exposure to improve the accuracy of relief features. It is believed that laminating reduces the gap between the mask and precursor so that curing radiation is less likely to scatter into areas of the precursor surrounding a transparent area of the mask. Zwadlo also teaches using a mask that includes a transparent substrate layer as a barrier. It is believed that laminating such a mask on a precursor prevents oxygen from reaching the plate precursor during exposure. In the presence of oxygen, some plate precursor materials require higher exposure levels to cure and thus image features can shrink in size, resulting in less accurate features.
U.S. Pat. No. 6,492,095 (Samworth) teaches using a pattern of opaque features in a mask to form a pattern of ink-carrying cells (holes) in solid relief areas to improve ink transfer to the printing substrate. Samworth teaches that the cell size should be small enough so that the aggregate volume of ink-carrying cells is less than that of the cells in the inking roller but big enough to form holes in the relief. Samworth suggests a suitable size is approximately 30 microns in diameter corresponding to a cluster of typical (e.g. 2400 DPI or approximately 10 micron) image pixels. Thus, in contrast with Gelbart, Samworth teaches deliberately creating holes in the relief media but only in areas of solid relief.
U.S. Pat. No. 6,731,405 (Samworth) extends the idea to also create ink-carrying cells in other halftone relief features according to the associated tonality. Samworth teaches using smaller or fewer ink-carrying cells in lower-tone features and to vary the size or quantity so that a greater aggregate cell volume is achieved for areas of higher tone than for areas of lower tone.
U.S. Patent Publication No. 2007/0002384 (Samworth et al.) teaches controlling ink film thickness on halftone dots by controlling the dimension of halftone dot relief features. For example, an approximately circular halftone dot will include at least one concentric ring of pixels that receives ink and one concentric ring of pixels that does not receive ink. This is in contrast with U.S. Pat. Nos. 6,492,095 and 6,731,405 (both to Samworth) which teaches creating ink-receptive cells in ink-receptive relief areas.
U.S. Patent Publication No. 2007/0002384 (Samworth et al.) also teach that the dimension of ink-receptive portions of halftone dots should vary to allow different thicknesses of film to be delivered for different tonalities. In particular, Samworth et al. teaches that near 50% tone, ink film thickness should be increased by increasing the dimension of ink-receptive portions of halftone features. Further, Samworth et al. teaches increasing the dimension (e.g. ring width) to make a smooth transition to a solid ink-receptive area for 100% tone.
U.S. Pat. No. 6,701,847 (Weichmann) teaches varying ink density on the printed substrate by superimposing a basic halftone image raster with a fine microraster to reduce the quantity of transferred ink. The microraster serves to reduce the area coverage of image areas (e.g. create holes in the printed image) to reduce the quantity of transferred ink. Weichmann further teaches varying the microraster to provide a gentle transition in reduced area coverage from a maximum amount at full tone to a minimum amount at some lower tone. Weichmann teaches, for example, the use of a checkerboard microraster with 5 micron by 10 micron holes arranged in a checkerboard pattern to achieve a 50% area coverage and corresponding ink density reduction.
Thus, it is clear from the prior art that creating a pattern of holes in halftone data can be used for a variety of purposes. It is not clear from the prior art why seemingly similar techniques produce significantly different results in the printing plate and printed image. It may be that some techniques produce different results for different printing processes. From empirical study of the state of the art of relief printing, however, it is clear that there is room for improvement.
For example, accurate representation of halftone relief features throughout the tonal range is still a challenge. In particular, it is desirable that relief features have relatively steep shoulders in order to resolve very fine features and to provide a precise delineation of relief boundaries. It is also desirable that printed ink densities range from a maximum amount for full tone image areas to minimal amounts in the extreme highlight tonal areas. It is also desirable that printed tonality vary with nearly a linear correlation to requested tonality. It is also desirable that ink be transferred with a uniform appearance in areas of consistent tonality.